Kernel#loop

loop { block }

Returns: nil or block's last expression · Updated March 13, 2026 · Kernel Methods

iteration loops infinite control-flow

loop is a built-in Kernel method that executes a block indefinitely until explicitly terminated with break. It’s useful when you need an infinite loop and want full control over when to exit.

Syntax

loop do
  # code to execute repeatedly
end

Parameters

loop takes no parameters. It accepts a block that will be executed repeatedly.

Examples

Basic infinite loop

counter = 0

loop do
  counter += 1
  puts counter
  
  break if counter >= 5
end

# Output:
# 1
# 2
# 3
# 4
# 5

Using next to skip iterations

counter = 0

loop do
  counter += 1
  
  next if counter.odd?
  
  puts counter
  break if counter >= 10
end

# Output:
# 2
# 4
# 6
# 8
# 10

Raising and rescuing to simulate retry logic

attempts = 0

loop do
  attempts += 1
  
  begin
    # Simulate a flaky operation
    result = rand(1..10)
    
    if result < 7
      raise "Failed attempt"
    end
    
    puts "Success on attempt #{attempts}!"
    break
    
  rescue => e
    puts "Attempt #{attempts} failed: #{e.message}"
    raise if attempts >= 3
  end
end

Common Patterns

Generating sequences

# Generate Fibonacci numbers
a, b = 0, 1

loop do
  puts a
  a, b = b, a + b
  break if a > 1000
end

Waiting for a condition

require 'timeout'

status = :pending

loop do
  # In real code, this would check an external resource
  status = :ready if rand > 0.7
  
  break :done if status == :ready
end

Reading from a stream until exhausted

# Read lines until EOF
loop do
  line = gets
  break if line.nil?
  puts line.upcase
end

Errors

LocalJumpError: If break is called outside a block, Ruby raises a LocalJumpError.
ArgumentError: loop does not accept arguments. Passing arguments will raise an ArgumentError.